(Mawrth Vallis is one of three candidate landing sites being studied for the ExoMars 2020 mission. The other sites are Aram Dorsum and Oxia Planum. Hypanis Vallis was considered until 2015 - it was downselected at the Third Landing Site Selection Workshop.))
The Mawrth Vallis region (approximately 22°N, 342°E) is located where the highlands of Arabia Terra meet the lowlands of Chryse Planitia. This area contains a large outflow channel known as Mawrth Vallis, which once discharged huge amounts of water onto the northern plains. The proposed ExoMars landing site is situated just south of this channel.
The region surrounding Mawrth Vallis contains one of the largest exposures of phyllosilicates (clay minerals produced by chemical weathering) detected on Mars. These deposits provide a unique opportunity to evaluate aqueous activity on early Mars and point to the possibility that habitable environments may have existed during the Noachian period (from the formation of the planet to perhaps 3.6 billion years ago). Other deposits and outcrops in the region show the highest degree of mineral alteration identified so far on Mars.
The channel floor of Mawrth Vallis also contains clay-rich layers, as does the nearby crater, Oyama. This indicates that the Mawrth Vallis outflow carved into ancient, clay-bearing, layered deposits. Later, smaller-scale fluvial processes eroded this material, especially on each side of the Mawrth Vallis channel and on the walls of Oyama crater.
The composition of the Mawrth Vallis clay-rich sediments shows vertical variation, with iron-rich clays at the bottom of the sequence and aluminium-rich clays present towards the top. This progression is evidence of different environmental conditions, indicating that the region once supported multiple, varied, wet environments.
A thin dark cap, up to 10 m thick and possibly of volcanic origin, overlies large parts of the Mawrth Vallis plateau. This dark material is younger than the phyllosilicates and does not seem to be hydrated. It probably once covered the entire region, but has since been eroded in many places.
The habitability potential of the Mawrth Vallis deposits is not clear, but the original sediments – possibly volcanic ashfall – could have hosted and preserved the signatures of microbial life if they were deposited in one or more bodies of water. On the other hand, if the volcanic material fell on land, it would not have been directly habitable. However, subsequent alteration of these deposits by water could have created potentially habitable niches to which viable living cells could have been transported.
Even if the deposits had only been wetted intermittently – for brief periods, but over a long time span – they could still have been colonised, provided viable life existed in their vicinity.
The higher latitude of the Mawrth Vallis site means that the landing ellipse for the ExoMars mission is larger than for the other sites (its major-axis is 170 km instead of 104 km; the minor-axis is 19 km), so it is more challenging to define a safe landing ellipse pattern. There are some areas of high land and steeper gradients which are potentially hazardous, but the dust coverage is low and the surface is generally quite smooth.
Further details about the landing site selection process can be found in the report of the Landing Site Selection Working Group: Recommendation for the Narrowing of ExoMars 2018 Landing Sites.